1. Field of the Invention
This invention relates generally to multimode engines capable of operating in multiple fueling modes, and, more particularly, relates to a method and apparatus for transitioning between fueling modes in such an engine so as to reduce engine speed fluctuation and/or other undesired responses to such transitions.
2. Discussion of the Related Art
So-called “multimode” engines are capable of operating in multiple fueling modes in that they are powered by different fuels or combinations of fuels depending, e.g., on the prevailing engine speed and load conditions. For example, a dual fuel engine can typically operate in two modes, namely, a “diesel-only mode” and a “pilot ignited gaseous fuel mode” or simply “pilot mode.” In the diesel-only mode, the engine is fueled solely by a liquid fuel, typically diesel fuel. In the pilot mode, the engine is fueled primarily by a gaseous fuel, such as natural gas or propane, which is ignited by a relatively small quantity or “pilot” charge of a liquid fuel, typically diesel fuel or engine lube oil.
Depending upon the particular engine utilized, there are typically at least two transition points within the operating range of a dual fuel engine. Specifically, the typical engine is operated in pilot mode except at the condition that the excess air ratio (lambda) of gas does not permit, such as, (1) very light load under all engine speeds and, (2) at high load, low speed conditions. The transition historically was triggered and controlled based solely as a function of speed and/or load without attempting to achieve a smooth transition. This relatively uncontrolled transition could lead to undesired speed fluctuations. For example, in a prior art dual fuel system, as the vehicle is driving up a hill while operating in pilot ignited gaseous fuel mode, the vehicle's engine speed may lug down sufficiently to trigger a changeover to diesel mode. An uncontrolled rapid switchover to diesel may cause a power surge and a resultant increase in vehicle speed back above the pilot mode transition speed for the prevailing load, whereupon the engine switches back to pilot mode and experiences a power drop. As a result, the vehicle speed may again drop below the transition speed with a resultant switchover to diesel-only mode. Hence, the engine may switch frequently and repeatedly between operating modes, resulting in noticeable speed surges and droops.
Some prior systems have recognized the problem identified above and have attempted to address it by taking the total energy content of the fuel(s) into account during the transition in an attempt avoid power surges and droops. For instance, U.S. Pat. No. 6,101,986 to Brown (the Brown patent) controls the delivery of diesel and gaseous fuel to the engine during transition between the pilot mode and the diesel-only mode to maintain the energy content of combined fuel charge at the desired value of the diesel fuel charge supplied at the end of the transition period. As a result, the quantity of diesel fuel progressively increases during the transition period, while the quantity of gaseous fuel progressively decreases. The process is repeated in a cycle-by-cycle basis until the actual diesel fuel quantity equals the desired quantity for diesel only operation, at which point the transition is considered complete.
A problem associated with prior techniques for controlling the transition between operating modes in a multimode engine is that simply maintaining the total fuel energy content constant during the transition period fails to take differences in combustion efficiency into account while air charge parameters remain unchanged. That is, (1) diesel fuel has a lower heating value and a lower stoichiometric air fuel ratio than gaseous fuel per unit fuel mass and, (2) combustion efficiency of pilot ignited gaseous fuel depends on excess air ratio of gas (gas lambda) and ignition timing. Simply increasing or decreasing gaseous fuel quantity may not achieve the desired effect because gas lambda may be outside of an optimal range for the selected gaseous fuel quantity. Existing airflow control devices are incapable of adjusting airflow to the cylinders rapidly enough to immediately obtain the optimum lambda for the selected quantity of the new fuel. As a result, the engine may still exhibit power surges and droops, even if total fuel energy content remains constant.
More recently, U.S. Pat. No. 7,270,089 to Wong proposed a more sophisticated technique in which at least one engine operating parameter other than total fuel energy content is taken into account in order to maintain a smooth transition between modes of a multimode engine. The parameter preferably comprises at least one of primary fuel excess air ratio (lambda) and ignition timing, and preferably is controlled in addition to total fuel energy content control. Lambda control is especially beneficial because it permits the control system to compensate for the engine's inability to substantially alter the instantaneous air mass in the combustion chamber during the transition period. For instance, during a transition from pilot mode to diesel-only mode, the controlled parameter typically comprises diesel lambda, and the controlling operation comprises setting a target or desired diesel lambda at a relatively high value at the beginning of the transition period and thereafter reducing diesel lambda during the transition period. In this case, the controlling operation may comprise determining a gas lambda of the gaseous fuel, determining a diesel lambda limit, and adjusting diesel fuel delivery to be at or above the diesel lambda limit. The diesel lambda limit preferably is initially determined based on the prevailing gas lambda and then adjusted downwardly on a cycle-by-cycle basis to a final value that is at or near the diesel smoke limit. The magnitude of adjustment in each cycle is preferably speed and/or time dependent.
The technique disclosed in the Wong patent works very well. However, it is not easily-implemented on a single point injection system in which the gas is introduced into the air supply system upstream of the air intake manifold via a mixer. In these systems, there can be a significant lag between the time that the gaseous fuel supply is initiated or terminated and the time that the gas reaches the first cylinders in the supply stream. Controlling fuel flow based only on total energy content, lambda, or other engine operating conditions without taking this delay into account can result in an unintended oversupply or undersupply of gaseous fuel. Depending on the available prevailing air flow ratio and resultant gas lambda, an unintended undersupply can lead to the lean limit of the engine's gaseous fuel supply being exceeded with the potential for misfire. An unintended oversupply can lead to temporary power surge.
The need therefore exists to provide a multimode engine that assuredly provides a smooth transition between operating modes using a simple, easy to implant strategy.
The need also exists to provide a method of providing a smooth transition between operating modes of a multimode engine, even if the engine is provided with single point gas injection.